In sailing terms the relative motion of a wind powered craft with regards to the wind direction may be referred to as sailing upwind, or sailing downwind. One sails downwind by positioning the sail to create as much drag as possible. This provides a wind pressure on the sail that pushes the craft along in the direction of the wind. For sailing downwind the sail configuration is not that critical. However, when sailing upwind or into the wind, the sail shape is more important. In order to sail upwind, the sails are positioned to generate aerodynamic lift in relation to the apparent wind. It is this lift that is translated into the driving force that propels the craft forward. When sailing upwind, drag is no longer beneficial, as it counteracts the resulting drive force. Therefore, it is important to developing sails that maximize lift while minimizing drag for sailing into the wind.
Three dimensional sails differ from conventional sails in that they have two surfaces of curvature rather than a single thin surface. These two surfaces create a pressure differential by forcing air to flow past them at different velocities, thus creating lift. As a result, high lift airfoils are asymmetrical and only generate lift efficiently in one direction. However, tacking in this manner is more difficult with a three dimensional sail due to its three dimensional fixed shape.
Accordingly, it is an object of the present invention to provide an improved three dimensional sail structure that is reversible as to its shape so as to facilitate lift in alternate directions.
Another object of the present invention is to provide a three dimensional sail apparatus that is reversible so as to assist in a “tacking” action.